MaineJay

So I'm not sure how great a winter threat this is, but in looking through the EPS members, there is support for some type of system moving through. I believe it's separate from the previous thread from Poc and the subsequent one from UTS. At least I hope so.

Everything from a western great Lakes track, to something sliding off the SE coast, and everything in between is represented.

Pattern: Persistent pattern of the past few days continues earlythis morning with vortex centered just north of Hudson Bay...anotherover eastern Asia....with ridging just off the west coast of NorthAmerica with early morning water vapor imagery revealing ashortwave helping re-establish troughing over the southwesternUnited States. Deterministic/ensemble guidance remains in strongagreement in maintaining this pattern through the comingweekend. Beyond this...there continues to be strong signals fora significant pattern change with Pacific ridge retrogradingwest and building...which allows significant troughing to takeshape over western North America with a weakening/eastward shiftof the Hudson Bay vortex. This favors building heights overeastern North America with a Bermuda high setup directing deepsouthwesterly flow along the eastern seaboard. The flow remainsprogressive through this period with no downstream blocking...so even this warm up does not look to remain with us for verylong. Primary forecast concerns through the long term forecastperiod include potential weekend snowfall...then the focus turnsto deep moisture plume along the eastern flank of developinglongwave trough and how much precipitation potential we havefrom this feature.

The Details...

Monday-Monday Night: High pressure will be centered just east of theregion to begin Monday with warm advection and top down saturationrapidly setting in as the day progresses. Thus...expect an increasein clouds with rain showers approaching from the south and westtowards evening. T9s moving to around the freezing mark should allow40s for areas outside of the mountains. Showery precipitation looksto arrive Monday night and will likely take the form of rain formost location...with the potential for a mix at the onset over themountains although even here temperatures aloft warm rapidly so thethreat will more likely be from pockets of trapped low levelcold air potentially leading to some freezing rain. Mild nightwith a non- diurnal trend likely /temps slowly rising throughthe night/ with 30s and potentially some 40s south.

Tuesday - Wednesday: Substantial run to run and model to modeldifferences develop by this time range with the EC/GFS/GGEM /andtheir ensemble counterparts/ all favoring a western trough...but very different evolutions along the eastern flank of thisfeature...reducing confidence. There is good confidence intemperatures moving well above normal for Tuesday and likelyWednesday with T8s at or above 10C. PWATs also increasedramatically as the Gulf of Mexico opens up. Looking at both theEC EPS and the GEFS...there is certainly enough overlap inguidance thinking to keep chance PoPs through this period...withthe greatest precipitation potential favored west as currentindications suggest that the best heavy rainfall signal will bewest of our region. Certainly something to keep an eye on forpotential hydro concerns. Otherwise...story will betemperatures...with good agreement that T9s move to near orabove 10C Tuesday...with some potential to be warmer still onWednesday. While this suggests the potential for 60s...have tobe careful with precipitation potential as well as decreasingconfidence... particularly on Wednesday. Still...boosting highsfor both days with 40s north and 50s south is a good startingpoint that can be adjusted as confidence grows.

I thought it might make sense to start a thread to document the upcoming cold. Not sure of the end date, suggestions welcomed.

EPS

Also, since there might be some waves that move along the edge of the cold, feel free to discuss potential for snow if there's no thread for it.

GYX

QUOTE

Beyond Christmas we begin to see signs of very cold air poisedto drop into the area. The core of a very cold Arctic low willbe sitting over south central Canada and southwest Hudson Bay,and the departure of the Christmas storm will begin to draw thiscold air southeastward into northern New England. With the freshcoat of snow across New England, expect this cold air mass toencounter little resistance in heading this way, reaching ourarea with full potency. The leading edge of this cold airarrives on Tuesday, with a stiff west northwest windaccompanying it. 925MB temperatures of -20C to -30C suggestsurface temperatures falling below zero Tuesday evening in thenorthern and eastern part of the area in spite of the low levelmixing. Very dry air mass, fresh snow pack, and some of thelongest nights of the year should allow nighttime temperaturesto fall below zero area wide, and to -20F or colder in thenorthern valleys Wednesday night and Thursday night. Daytimehighs may not rise above zero in northern and eastern areas fora few days! It is likely that a wind chill headline of some sort(Advisory or Warning) will be needed for part of the area whenthis cold air arrives, most likely in the Tuesday to Wednesdaytime period when winds are still fairly strong.

Last 5 ECMWF runs. Last run is valid at hour 114, this is when models start to become a bit more reliable. Unfortunately, there appears to be subtle phasing details which have significant sensible weather effects. So I'll be watching for the potential for stream interactions. Or even a brief cutting off.

A SPLIT FLOW REGIME IS NOTED DURING THE PERIOD WITH A FEW PRIMARYFEATURES TO NOTE. ON WEDNESDAY MORNING...A STRETCHED OUT STRIP OFVORTICITY SLIDING THROUGH THE CENTER OF THE COUNTRY WILL CONTINUETO ACCELERATE EASTWARD REACHING THE MID-SOUTH BY THE FOLLOWINGMORNING. DEPENDING ON THE DEPTH OF THIS SHORTWAVE...SOME WAVES OFLOW PRESSURE SHOULD DEVELOP ALONG THE EXITING WAVY FRONTALBOUNDARY. THE PRIMARY SURFACE CYCLONE MAY STRENGTHEN OVER THEWESTERN ATLANTIC BUT SHOULD REMAIN FAR ENOUGH OFF THE EASTERNSEABOARD. LOOKING BACK TOWARD THE WEST COAST...A POWERFUL UPPERLOW WILL DROP DOWN FROM THE GULF OF ALASKA WITH FORECAST 500-MBHEIGHT ANOMALIES ON THE ORDER OF 2.5 TO POSSIBLY 3 STANDARDDEVIATIONS BELOW AVERAGE VIA THE 00Z GFS. THIS CLOSED CIRCULATIONSHOULD REACH THE NORTHERN CA/PACIFIC NORTHWEST COAST BY LATEWEDNESDAY WHILE GRADUALLY EVOLVING INTO MORE OF AN OPEN WAVETHEREAFTER. A LIKELY PROGRESSIVE STACK OF SHORTWAVES SHOULDACCELERATE TOWARD THE GREAT PLAINS/MISSISSIPPI RIVER VALLEY BYNEXT WEEKEND. LASTLY...AMPLIFICATION IN THE NORTHERN STREAM WILLPOTENTIALLY DRAW A SIGNIFICANT SHOT OF ARCTIC AIR TO SECTIONS OFTHE CENTRAL/EASTERN U.S. FROM THURSDAY ONWARD. MODELS EXHIBITEXTREME DIFFERENCES WITH THIS FEATURE WHICH WILL BE NOTED IN THEFOLLOWING SECTION. LASTLY...AMPLIFIED FLOW RETURNS TO THE EASTERNPACIFIC TOWARD THE LATTER HALF OF NEXT WEEKEND.

REGARDING THE INITIAL STRUNG OUT SHORTWAVES MOVING TOWARD THELOWER TENNESSEE VALLEY...MODELS HAVE BEEN TRENDING TOWARD ASLIGHTLY HIGHER AMPLITUDE WAVE. THIS WOULD INDUCE A MODEST REGIONOF LOW PRESSURE ALONG THE FRONT WHICH COULD DEEPEN WHILE MOVINGOUT OVER THE WESTERN ATLANTIC. SHIFTING FOCUS TO THE WESTCOAST...MULTI-MODEL CYCLE TRENDS SHOW AN EASTWARD JOG WITH THEPOSITION OF THE EASTERN PACIFIC UPPER LOW. IT APPEARS THAT THELARGEST DIFFERENCES ARE WITH REGARD TO THE STRENGTH OF THEDOWNSTREAM RIDGE WITH MANY OF THE 12Z ECMWF ENSEMBLE MEMBERSFAVORING A HIGHER DEGREE OF AMPLITUDE. AS THE PROGRESSIVESHORTWAVE CONTINUES ITS MARCH TOWARD THE CENTER OF THECOUNTRY...THE SURFACE PATTERN BECOMES COMPLICATED BY EFFECTS FROMTHE AMPLIFICATION IN THE NORTHERN STREAM. WITH THE PAST FEW RUNSOF THE ECMWF SHOWING MUCH GREATER SOUTHWARD ELONGATION OF THISTROUGH...A ROBUST SURFACE ANTICYCLONE SETS UP OVER THE UPPERMIDWEST BY FRIDAY MORNING. MEAN SEA LEVEL PRESSURES WOULD BEAROUND 1036-MB ON 10/1200Z BASED ON THESE ECMWF RUNS.MEANWHILE...THE 00Z/18Z/12Z GFS WOULD SUPPORT A SURFACE CYCLONE INTHIS EXACT POSITION WHICH LEADS TO QUITE A FORECAST CHALLENGE. THE00Z GFS FROM YESTERDAY WAS THE LAST RUN IN ITS SUITE WHICHDEPICTED THE DEGREE OF AMPLIFICATION SHOWN BY THE ECMWF. UNTIL THE00Z CMC ARRIVED LOOKING MORE LIKE THE 12Z ECMWF...THE CMC/UKMETHAD NOT OFFERED MUCH RECENT SUPPORT FOR THIS AMPLIFIED SCENARIO.COMPARISON OF MULTI-DAY ENSEMBLE SPAGHETTI PLOTS...WITH THEEXCEPTION OF THE ECMWF ENSEMBLE MEMBERS...THE CMC/GEFS HAVE BEENWAFFLING AROUND ALL OVER THE PLACE. SUFFICE TO SAY...A LOWCONFIDENCE FORECAST IS AT HAND HERE. FINALLY...ALL GLOBALENSEMBLES AGREE ON A RETURN OF LONGWAVE TROUGHING TO THE EASTERNPACIFIC BY NEXT WEEKEND....A GENERAL REPEAT OF WHAT IS IN PLACE ONDAY 3/WEDNESDAY.

GIVEN THE RUN-TO-RUN CONSISTENCY EXHIBITED BY THE ECMWF AND ITSENSEMBLE MEAN...DECIDED TO LEAN MORE TOWARD ITS SOLUTIONTHROUGHOUT THE PERIOD. AMONG THE GFS CYCLES...FAVORED THE PREVIOUS00Z RUN AS IT HAD THE MOST IN COMMON WITH THE ECMWF WHILE ALSOSTAYING SLIGHTLY CLOSER TO CONTINUITY. GENERALLY TOOK AN EVENSPLIT OF THESE TWO SOLUTIONS THROUGHOUT THE PERIOD WHILE QUICKLYINCORPORATING A MORE ENSEMBLE BASED APPROACH AFTER FRIDAY. DIDTHROW A TOKEN AMOUNT OF THE 18Z GFS IN THE MIX TO ACCOUNT FOR ANYSHIFT TOWARD A MORE DE-AMPLIFIED SCENARIO.

...SENSIBLE WEATHER/THREATS...

EXCEPT FOR THE SOUTHEASTERN U.S. AS WELL AS THE DESERTSOUTHWEST/FOUR CORNERS REGION...MUCH OF THE COUNTRY WILL BEENTRENCHED IN BELOW AVERAGE TEMPERATURES THROUGHOUT THE FORECAST.OF COURSE THIS IS HIGHLY DEPENDENT ON THE MODEL CHOICE WITH THEGFS/ECMWF DIVERGING BY ALMOST 30 DEGREES BY DAY 5/FRIDAY. GIVEN ACOLD SOLUTION WAS FAVORED...EXPECTED DEPARTURES FROM NORMAL WILLBE AROUND 10 TO 20 DEGREES BELOW AVERAGE AS THE MODIFIED ARCTICAIR WORKS ITS WAY INTO THE CENTRAL/EASTERN U.S. THE COLD SPOTSHOULD BE FROM NORTHEASTERN MONTANA THROUGH NORTH DAKOTA AND INTONORTHERN MINNESOTA WHERE LOWS WILL STRUGGLE TO ESCAPE THE SINGLEDIGITS UNTIL FRIDAY. AT THIS TIME...IT DOES NOT APPEAR DAILYTEMPERATURE RECORDS WILL BE BROKEN.

THE SLOW-MOVING FRONT STRETCHING FROM THE ARKLATEX INTO THECAROLINAS WILL BE AN INITIAL FOCUS FOR WIDESPREAD RAINFALL. ALLTHIS ACTIVITY WILL SHIFT SOUTHWARD WITH CONDITIONS GRADUALLYIMPROVED TOWARD THE END OF THE WORK WEEK. ACROSS THE WESTCOAST...HEAVY PRECIPITATION ACCOMPANIED THE APPROACHING CLOSED LOWWILL IMPACT NORTHERN CA UP ALONG THE PACIFIC NORTHWEST COAST. THEACTIVITY IS FORECAST TO SHIFT TOWARD THE UPPER INTERMOUNTAIN WESTWITH ENOUGH COLD AIR TO ALLOW SNOWFALL OVER THE HIGHER TERRAININCLUDING THE CASCADES...BITTERROOTS...SAWTOOTH...AND TETONS.FARTHER DOWNSTREAM...SOME WARM ADVECTION SHOWERS ARE POSSIBLE OVERTHE CENTRAL U.S. ALTHOUGH IT IS UNKNOWN HOW MUCH MOISTURE RETURNTHERE WILL BE. AND ACROSS INTERIOR NEW ENGLAND...WHILE MOISTUREWILL LIKELY BE MEAGER...SOME ACCUMULATING SNOWS ARE POSSIBLEACCOMPANYING THE POTENTIAL ARCTIC INTRUSION.

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...Overall 500 mb hemispheric pattern in the models looks fairlysimilar to previous runs, with large scale troughing overcentral providing mostly zonal flow to the nrn tier of CONUS.Initially will see more of a SW flow, which will eventuallytranslate to a cyclonic tendency later in the forecast period.Still looking at temps running normal, but skewing slightly coolthru most of the week, with a possible brief shot of Decemberair Fri-Sat.

Digging trough extending equatorward from deeper closed low nearHudson will swing SE across the CWA Fri into Sat. This couldproduce some rain or snow showers in the mtns and foothills Thunight into Fri and a shot of below normal air moving in Fri-Sat.00Z Euro backing off a bit on the cold, but still look for belownormal temps with highs FR-Sat in the upper 30s to mid 40s andlows Fri night in the 20s, and maybe some teens in the mtns. Thecold air will be quick to leave with temps climbing back tonear normal on Sunday.